1. Field of the Invention
This invention relates to a process and an apparatus for producing an optically uniform, i.e., optically homogeneous, highly transparent coating or the like from a mixture of a plurality of components which can be flow-molded and which react to form a polyurethane.
2. Description of the Prior Art
In certain cases, objects comprised of plastic material must satisfy extremely stringent specifications with regard to the uniformity of the plastic material. For example, certain coatings, films, or laminating sheets are required to be free of optical defects and distortion. Such highly transparent coatings, films, and sheets are employed, e.g., as films to protect against fragmentation, on window panes comprised of silicate glass; or as coatings to improve the abrasion resistance of plastic objects. Examples of such safety glass window panes and abrasion-resistant plastic sheets, as well as examples of production of such coatings and the like from specific polyurethane materials, are described, e.g., in Ger. OS No. 20 58 504, Ger. AS Nos. 22 28 299, and 26 29 779.
In reactive flow-molding processes of this type, employing a plurality of components, the homogeneity and thus the optical quality of the polyurethane sheet or film is basically determined by the mixing operation which immediately precedes the molding of the said coating. As soon as the reactants come into contact, the polyaddition reaction effectively begins. Since the reacted portions of the mixture have a different viscosity from that of the reaction components (i.e., the unreacted reaction mixture), striae form within the molding mass which are visible in the sheet or film when the reaction is finished. Therefore, it is important to produce a mixture of the reaction components which is as homogeneous as possible, in the shortest possible time, in order to bring about uniform reaction in the molded mass so as to avoid formation of striae. There is an additional difficulty presented in mixing the reaction components in the case of polyurethane, in that the two components, viz. the polyol and the isocyanate, have substantially different viscosities, differing, according to measurements, by a factor of 4 to 8 or more.
Therefore, a need continues to exist for a process for mixing two or more components of a system of a plurality of components, in particular components of a reaction mixture containing and/or resulting in a polyurethane molding resin, whereby in the process an optically homogenous mixture is produced in the minimum time possible following the combination of the reaction components which satisfies the stringent requirements of physical and chemical homogeneity.